The present invention relates to an AC generator (i.e., so-called alternator) for an automotive vehicle, which is preferably installed in a passenger vehicle or a truck or the like.
To reduce the aerodynamic resistance in a traveling condition, a vehicle body tends to be formed into a slant nose shape. Securing a sufficient residential space for a passenger compartment is earnestly demanded. To satisfy these requirements, engine rooms of automotive vehicles have been becoming so narrow and crowded that only a limited space is available for installing an AC generator. Meanwhile, to improve fuel economy, rotation of an engine tends to be reduced. Correspondingly, rotation of an AC generator is lowered. On the other hand, there is a need for increasing electric load for safety control devices etc. Thus, improving the generating ability of the AC generator is strongly required. In other words, a compact, powerful, and non-expensive automotive AC generator is required.
To this end, the power output may be increased by reducing the winding resistance and air gaps or by increasing the exciting current. However, this will induce heat generation at various components constituting the AC generator. Especially, a rectifying element (i.e., diode) is subjected to a large temperature increase. The rectifying element is generally used to rectify electric power when generated by a stator from AC voltage into DC voltage. To secure the reliability of this rectifying element, it is necessary to cool the cooling fin satisfactorily within a limited space. To improve the cooling performance, it is of course possible to enlarge the cooling fan so as to increase an amount of cooling air. However, solely increasing the cooling air volume will result in an increase of noise.
FIG. 4 shows an automotive AC power unit for a conventional automotive AC generator.
Cooling fins 511 and 512, used for cooling the rectifying elements, are disposed on a rear frame. An IC regulator 7 and a connector casing 8 are disposed at point-symmetrical positions with respect to the cooling fins 511 and 512, respectively. A brush holder 6 is disposed at a portion surrounded by them. An appropriate clearance is provided at a radial-inner end and a radial-outer end of each of the cooling fins 511 and 512. More specifically, as shown in FIG. 5, the radial-inner end of each cooling fin 511 or 512 is spaced from a slip ring protector 91 with a radial clearance while the radial-outer end is spaced from a rear cover 92 with another radial clearance. The cooling fins 511 and 512 have circumferential ends which are partly brought into contact with neighboring components so as to provide open spaces 93.
A central portion of each of the cooling fins 511 and 512 is sufficiently cooled by cooling air, as the cooling air is introduced from an opening provided on the rear cover 92 and flows along the surfaces of these cooling fins 511 and 512, as shown in FIG. 5. However, the cooling air does not flow at the above-described contacted end portions of the cooling fins 511 and 512. Meanwhile, the cooling air flow resistance is lowered significantly at the opened end portions of the cooling fins 511 and 512, compared with other portions where components are disposed. Thus, the introduced cooling air may directly go out through the opened end portion without interacting with the cooling fins 511 and 512. Accordingly, cooling performance is not uniformed among a plurality of diodes 513 and 514 arranged in a circumferential direction on the cooling fins 511 and 512. More specifically, the diodes 513 and 514 positioned at the circumferential ends of the cooling fins 511 and 512 are not satisfactorily cooled by the cooling air, compared with the diodes 513 and 514 positioned at the circumferential centers of the cooling fins 511 and 512.
U.S. Pat. No. 4,952,829 discloses a cooling fin for a rectifying element which has a sectorial opening at a radial-outer end of a brush holder. This arrangement will encounter with the above-described problem that the rectifying elements positioned near the opening are not sufficiently cooled because the cooling air is directly guided to the opening.